This invention relates in general to burners and more particularly to a gas burner for heating an airstream from within the airstream.
Perhaps the most efficient method of heating an airstream is to maintain combustion directly in the airstream, using the airstream itself for the combustion air. Gaseous fuels such as natural gas are about the only fuels suitable for this purpose since they usually burn completely and leave no residue or other obnoxious products of combustion to contaminate the airstream. However, maintaining a flame in a fast moving and perhaps turbulent airstream is not easily done, and indeed requires special burner configurations. Some of these burners operate only at high flame settings, while others can be turned down, but heretofore the turn down ratio has been relatively modest on the order of about 10:1.
Moreover, in burners having the capability of modulating the flame, sensing the flame has been a troublesome problem because the flame can assume many different positions, depending on the amount of gas introduced into the burner and the turbulence of the combustion air. For example, when the gas supply is turned down to the minimum necessary to sustain the flame, the base of the flame is located just beyond the gas orifice and the flame tip is located not much beyond the base. On the other hand, when the gas supply is at high volume, the base of the flame is located substantially beyond the gas orifice, and the flame possesses considerable length, extending beyond the burner in some instances. The transposition of the flame base at high setting results primarily from the fact that at high setting the gas-air mixture adjacent to the burner is far too rich to support combustion. Only when the gas becomes sufficiently mixed with the air does combustion occur and at high setting this is substantially beyond the burner orifice. Thus, it is difficult to position a flame probe or rod which will sense both high and low flames. In this same vein, the flame will move off of the flame probe, if the burner is over-fired or if there is insufficient combustion air. Of course, a flame probe or rod is necessary to avert the introduction of raw gas into the airstream should the flame become extinguished within the burner.